Preheating oxygen for injection into blast furnaces

ABSTRACT

A side stream of hot blast air is used to preheat oxygen at a heat exchanger. The resultant hot oxygen is injected into a tuyere of a blast furnace with pulverized or granular coal. The cooled side stream may be recombined with the hot blast air for injection into the tuyere, fed to the stove as part of the cold blast air, or fed to stove for combustion with blast furnace gas.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 61/735,181 filed Dec. 10, 2012, the disclosure of which is incorporated herein by reference.

BACKGROUND

1. Field of the Invention

The present invention relates to blast furnaces and methods and systems for preheating oxygen for injection into a blast furnace.

2. Related Art

Blast furnaces are the primary source of high-purity iron for steelmaking. High-purity iron is required for the manufacture of the highest quality steels which must have minimal levels of detrimental elements, like copper, which are difficult to remove chemically from steel. Blast furnaces are also used in the production of other metals such as ferromanganese and lead.

Traditionally, metallurgical coke has been the primary fuel and the source of the reducing gas consumed in the blast furnace process. Coke, fluxes and ore, such as iron ore, are charged in layers at the top of the furnace, and a hot air blast is blown into the bottom of the furnace. The air reacts with the coke, generating heat for the process and producing a reducing gas which preheats the coke, fluxes and ore, and converts the iron ore to iron as it flows up through the furnace. The gas exits the top of the furnace and is used in part as a fuel to preheat the air blast.

Metallurgical coke is formed by heating coal in the absence of air, driving off the more volatile components of coal. Many of these volatile components are an environmental and health hazard, and cokemaking in recent years has become increasingly regulated. The cost of complying with these regulations has raised cokemaking operating costs and increased the capital required for new cokemaking facilities. As a result, the supply of coke is shrinking and prices are rising. These factors have led blast furnace operators to decrease the amount of coke they use and to inject large amounts of alternate fossil fuels into the hot air blast supply to the furnace as a substitute. The most common fossil fuels injected are pulverized coal, granular coal, and natural gas. Either pulverized or granular coal is preferred for economic reasons.

Coke is preheated by the reducing gas as the gas flows up the furnace. In contrast, the coal is injected at ambient temperature. Accordingly, the addition of coal into the blast air supply adds a thermal load to the furnace which does not occur when only coke is used as the fuel. Operators of blast furnaces have addressed this problem by adding oxygen to the blast air and this has provided some benefit.

One method of introducing oxygen into a blast furnace is to enrich air with oxygen before heating the oxygen-enriched air at the hot blast stove to a temperature of around 1200-1300° C. In regenerative stoves, the level of oxygen enrichment is rather limited in this method due to safety reasons as some amount of combustible substances remain in the stove after regeneration.

Another method of introducing oxygen into a blast furnace is to injection oxygen from a lance located very close to the outlets of tuyeres. In contrast to the first method, the air can safely enriched to much higher levels. However, even with oxygen addition, blast furnace operation at higher fossil fuel injection levels has not been achievable because of blast furnace operating problems related to poor or incomplete combustion of injected coal. This is because the ambient temperature of the oxygen will create locally cool regions which delay combustion of the coal. Since the blast air is moving at a high velocity (as much as 120 m/s or greater), the injected oxygen is also moving very quickly. As a result, the locally cool regions are not heated to a high enough temperature to allow complete carbon combustion.

U.S. Pat. No. 6,090,182 identified the issue of incomplete combustion and proposed the injection of hot oxygen as a solution to the reduced coke savings realized through injection of ambient oxygen. It discloses that the temperature of the oxygen jet exceeds that of the blast air stream and is generally within the range of from 1200 to 1650° C. It allows any suitable means for establishing the defined hot oxygen jet.

U.S. Pat. No. 5,266,024 and U.S. Pat. No. 8,105,074 proposed a specific way of preheating the oxygen to be injected. A small amount of gaseous fuel is combusted with oxygen and the products of combustion are mixed with oxygen. U.S. Pat. No. 8,105,074 discloses that a mixture having above 80% of oxygen with a balance of CO₂ and H₂O can be obtained. This method suffers from the drawback of consuming too much fuel for preheating the oxygen. For a large blast furnace, the amount of injected oxygen can reach 80,000 Nm³/hr or more which would require about 10,000 Nm³/hr of natural gas. This is comparable to a burner having a power of about 100 MW. When the price of natural gas is high, this solution is uneconomical because the price of the natural required for preheating the oxygen can severely diminish or negate any coke savings.

Thus, there is a need for a method of preheating oxygen for injection into a tuyere of blast furnace with pulverized or granular coal that is more economical than conventional techniques for preheating such oxygen.

SUMMARY

There is disclosed a method for heating oxygen for injection into a tuyere of a blast furnace. The method includes the following steps. Cold blast air is fed to a first stove to produce a stream of hot blast air. The stream of hot blast air from the blast air stove is divided into a hot side stream of hot blast air and a remaining stream of hot blast air. A stream of industrially pure oxygen is heated at a heat exchanger through heat exchange with the hot side stream to yield a heated oxygen stream and a cooled side stream. Pulverized or granular coal is injected and the heated oxygen stream into the tuyere for combustion of the injected coal and oxygen.

There is also disclosed a blast furnace installation utilizing hot oxygen tuyere injection, comprising: a first stove receiving cold blast air and producing a stream of hot blast air; a heat exchanger adapted and configured to heat ambient oxygen through heat exchange with a portion of the stream of hot blast air; and a blast furnace including a tuyere that is adapted and configured to receive hot blast air, pulverized or granular coal, and also hot oxygen from the heat exchanger.

Either or both of the method and installation may include one or more of the following aspects:

-   -   the hot side stream is diluted with ambient air upstream of the         heat exchanger.     -   the cooled side stream is combined with the remaining stream.     -   the cooled side stream is fed to a second stove as part of         combustion air fed to the second stove.     -   the cooled side stream is fed to the first stove as part of cold         blast air that is heated at the first stove.     -   the tuyere is adapted and configured to also receive the portion         of hot blast air that is cooled at the heat exchanger.     -   at a point upstream of the tuyere, a line conveying the hot         blast air to the tuyere receives the portion of hot blast air         that is cooled at the heat exchanger.     -   a second stove receives the portion of the stream of hot blast         air that is cooled at the heat exchanger that is adapted and         configured to combust such cooled portion with blast furnace         gas.     -   the first stove receives the portion of the stream of hot blast         air that is cooled at the heat exchanger as part of the cold         blast air.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects of the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

FIG. 1 is a schematic of the inventive process with optional dilution of the side stream of hot blast air.

FIG. 2 is an embodiment of the process of FIG. 1 where the cooled side stream is combined with the remaining stream of hot blast air.

FIG. 3 is another embodiment of the process of FIG. 1, where the cooled side stream is fed to the stove.

FIG. 4 is yet another embodiment of the process of FIG. 1, where the cooled side stream is fed to the stove.

DESCRIPTION OF PREFERRED EMBODIMENTS

Oxygen may be preheated before injection into a tuyere of a blast furnace along with coal without having to consume additional fuel solely for the purpose of heating the oxygen. In this manner, the economic disadvantage of the proposal disclosed by U.S. Pat. No. 5,266,024 and U.S. Pat. No. 8,105,074 may be avoided.

As best illustrated in FIG. 1, cold blast air 1 is fed to a first blast air stove 2′ where it is heated up to produce a stream of hot blast air 3 with a temperature of about 1200° C. At the same time, blast furnace gas 4 is fed to a second blast air stove 2″ to regenerate the checker material. The stream of hot blast air 3 is divided into a hot side stream of hot blast air 5 and a remaining stream of hot blast air 6. The hot side stream 5 and an ambient oxygen stream 10 are fed to a heat exchanger 9 where the hot side stream 5 is cooled to produce a cooled side stream 13 and the ambient oxygen stream 10 is heated to produce a stream of hot oxygen 11. Pulverized or granular coal 14, the stream of hot oxygen 11, and the remaining stream of hot blast air 6 are fed or injected into the tuyere to combust the coal and oxygen.

The temperature of the hot oxygen stream 11 is limited by the relative amounts of ambient oxygen in stream 10 and hot blast air in stream 5 fed to the heat exchanger. In other words, a greater flow of hot blast air in stream 5 will lead to a higher temperature in stream 10, and vice versa. Typically, somewhere around 10% of the hot blast air stream 3 is devoted to the hot blast air side stream 5 with the balance devoted to the remaining stream 6. The temperature of the hot oxygen stream 11 is also limited by the ability of the material (comprising the oxygen side) of the heat exchanger 9 to withstand a hot oxidative attack. A greater selection of materials is available for the portion of the heat exchanger 9 exposed to hot oxygen if the temperature of the hot blast air of side stream 5 is first reduced before introduction into the heat exchanger 9. This temperature reduction may be achieved through dilution with a stream of ambient air 8.

Starting from a typical temperature of about 1200° C., the side stream of hot blast air 5 may be cooled at the heat exchanger 9 to a temperature of around 1100° C. If the side stream 5 is optionally diluted with ambient air, its temperature may be lowered to around 700° C. At the heat exchanger 9, such a diluted stream 5 is further cooled through heat exchange with the ambient temperature oxygen in stream 10 to a temperature of around 450° C. Regardless of whether the side stream 5 is diluted with ambient air, the flow rates and heat exchanger proportions are selected in routine fashion to result in a temperature of about 600° C. for the hot oxygen stream 11. Taking safety into consideration, a temperature of about 600° C. represents a typical upper limit for piping conveying the hot oxygen in stream 11 to the tuyere 12.

A significant amount of thermal energy remains in the cooled side stream 13. As best shown in FIG. 2, the cooled side stream 13 may be combined with the remaining stream of hot blast air 6 and injected into the tuyere 12. After combination, the hot blast air temperature may drop by about 50° C. However, in comparison to the flame temperature of the combusting coal in the case where the oxygen is instead heated up to around 600° C. by other means (and not by practice of the invention), the adiabatic flame temperature of the combusting coal resulting from practice of the invention will not be significantly different. This is because the thermal content lost from the stream of hot blast air 3 has been shifted to the hot oxygen stream 11. The advantage of such a process is that by transferring some of the heat from the blast air to the oxygen, the combustion efficiency of the injected coal increases without sacrificing any heat overall.

Alternatively, and as best illustrated in FIG. 3, the cooled side stream 13 may be combusted with blast furnace gas 4 at the second stove 2″. In this manner, the thermal content of the cooled side stream 13 may be transferred to the checker material in the second stove 2″ after combustion with the blast furnace gas 4 and used to preheat the cold blast air after regeneration of the checker material.

In another alternative and as best shown in FIG. 4, the cooled side stream 13 may instead be fed to the first stove 2′ where it is heated with the cold blast air 1 to provide the stream of hot blast air 3. In this manner, the thermal content of the cooled side stream 13 is utilized to produce the hot blast air in stream 3.

Preferred processes and apparatus for practicing the present invention have been described. It will be understood and readily apparent to the skilled artisan that many changes and modifications may be made to the above-described embodiments without departing from the spirit and the scope of the present invention. The foregoing is illustrative only and that other embodiments of the integrated processes and apparatus may be employed without departing from the true scope of the invention defined in the following claims. 

What is claimed is:
 1. A method for heating oxygen for injection into a tuyere of a blast furnace, comprising the steps of: feeding cold blast air to a first stove to produce a stream of hot blast air; dividing the stream of hot blast air from the blast air stove into a hot side stream of hot blast air and a remaining stream of hot blast air; heating a stream of industrially pure oxygen at a heat exchanger through heat exchange with the hot side stream to yield a heated oxygen stream and a cooled side stream; and injecting pulverized or granular coal and the heated oxygen stream into the tuyere for combustion of the injected coal and oxygen.
 2. The method of claim 1, further comprising the step of diluting the hot side stream with ambient air upstream of the heat exchanger.
 3. The method of claim 1, further comprising the step of combining the cooled side stream with the remaining stream.
 4. The method of claim 1, further comprising the step of feeding the cooled side stream to a second stove as part of combustion air fed to the second stove.
 5. The method of claim 1, further comprising the step of feeding the cooled side stream to the first stove as part of cold blast air that is heated at the first stove.
 6. A blast furnace installation utilizing hot oxygen tuyere injection, comprising: a first stove receiving cold blast air and producing a stream of hot blast air; a heat exchanger adapted and configured to heat ambient oxygen through heat exchange with a portion of the stream of hot blast air; and a blast furnace including a tuyere that is adapted and configured to receive hot blast air, pulverized or granular coal, and also hot oxygen from the heat exchanger.
 7. The installation of claim 6, wherein the tuyere is adapted and configured to also receive the portion of hot blast air that is cooled at the heat exchanger.
 8. The installation of claim 6, wherein, at a point upstream of the tuyere, a line conveying the hot blast air to the tuyere receives the portion of hot blast air that is cooled at the heat exchanger.
 9. The installation of claim 6, further comprising a second stove that receives the portion of the stream of hot blast air that is cooled at the heat exchanger and that is adapted and configured to combust such cooled portion with blast furnace gas.
 10. The installation of claim 6, wherein the first stove receives the portion of the stream of hot blast air that is cooled at the heat exchanger as part of the cold blast air. 